Method and system of treating a stainless steel strip, especially for a pickling treatment

ABSTRACT

A method for treating a steel strip uses a treatment liquid in a treatment station having at least a spray section, an immersion section, and a common collector for the liquid, the steel. strip including stainless steel, top surface, and a bottom surface, and being continuous, oriented substantially horizontally, both in its longitudinal and transverse directions, involves transporting the steel strip continuously through the station in a transport direction parallel to the steel strip&#39;s longitudinal direction, first, spraying the liquid, using one or more spray nozzles, onto the steel strip&#39;s top and bottom surfaces while in the spray section and, second, immersing the steel strip in the liquid while in the immersion section, wherein, while treating the steel strip, continuous pumping the liquid out of the common collector and through both the spray section and the immersion section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C. §371 of International Application No. PCT/EP2015/078314, filed on Dec. 2, 2015, and claims benefit to Europe Patent Application No. 14 195 952.8, filed on Dec. 2, 2014. The International Application was published in English on Jun. 9, 2016, as WO 2016/087494 A1 under PCT Article 21(2).

FIELD

The present invention relates to a method for treating a steel strip especially for a pickling treatment of the steel strip, by means of a treatment liquid in a treatment station, the treatment station comprising a treatment tank.

BACKGROUND

The presence of oxide scale on the surface of steel strip is formed during high temperature processing. The term oxide scale generally refers to the chemical compounds of iron and oxygen, as well as the chemical compounds of iron alloying elements, e.g. chromium and oxygen, formed on the surface of the steel by exposure to air while the metal is at an elevated temperature. Chemical compounds thus formed include iron oxides, such as FeO, Fe₂O₃ and Fe₃O₄, oxides of alloying elements such as CrO₃, NiO, SiO₂ and complex oxide spinals like FeCr₂O₄, NiFe₂O₄, Fe₂SiO₄ and others. During annealing, stainless steel grades are heated up to a certain temperature (850-1150° C. depending on steel grade) and are kept at this temperature for some time to soften the metal in order to release the work hardening induced by hot and cold rolling. A uniform grain structure is obtained depending on the annealing temperature, and oxide scale is formed on the surface. Underneath the surface of stainless steel, a chromium-depleted zone is formed, which is different for austenitic, ferritic and duplex stainless steel grades.

Processes are required in the stainless steel industry to remove that oxide scale and the chromium depleted layer to obtain technological products, but that has to be achieved with a minimum loss of base material. Pickling is the process of chemically removing of oxide scale from the surface of a metal by the action of water solution of inorganic acids. The stainless steel is widely pickled in diluted sulfuric or hydrochloric acid. For pickling of the stainless steel, a mixture of nitric acid and hydrofluoric acid is widely applied. The rate of pickling is affected by numerous variables, including the steel-based constituents and type and adherence of oxide to be removed. Pickling solution temperature, acid concentration, reaction product concentration, turbulence flow conditions, immersion time and presence or absence of inhibitors and accelerators influence the rate of acid attack. Because of production factors including pickling speed, quality and efficiency, as well as reduced attack of HCl on base metal, hydrochloric acid has effectively displaced sulfuric acid as the acid of choice in industrial large-scale pickling lines for stainless steel. While the rate of pickling increases in direct proportion to the concentration of the acid, the influence of temperature is much more pronounced. On the other hand, certain metals, such as cooper, chromium and nickel, retard the rate of pickling when they occur in the steel base, since the scale bearing these alloying metals inhibits acid attack. Elements like aluminum and silicon form refractory-type oxides, which in turn lower the solubility rate of the oxide in the pickling acid. The thickness of the oxide scale varies considerably with practice in rolling mills. E.g. loose coiling permits greater atmospheric penetration into the wraps, with corresponding heavier oxide formation on the edge areas. In addition, coiling temperature affects the adherence of the oxide and determines how easy or difficult it is to remove. The lower coiling temperatures makes oxide removal easier, at higher coiling temperatures longer pickling times are required. E.g. at the coiling temperature of 750° C., double pickling time is required compared to the coiling temperature of 570° C.

Like with carbon steel, the stainless steel also oxidizes following hot rolling and coiling. The oxide scale layer formed on the surface of the hot rolled stainless steel strip contains the alloying elements and is very tightly adhering on the surface, which makes the de-scaling or pickling of stainless steel very difficult as compared to the carbon steels. To achieve efficient and thorough surface oxide removal from a stainless steel strip, more severe processing techniques must be used which substantially increase processing time and operational costs. Frequently, to effect complete oxide scale removal, chemical pickling of stainless steel strip must be preceded by mechanical de-scaling, e.g. by shot-blasting and/or scale breaking. Often nowadays additional methods of Pre-Pickling is applied to soften the oxide scale. E.g. hot rolled stainless steel is conventionally pre-pickled in hot sulfuric acid and the cold rolled stainless steel is pre-pickled electrolytically in neutral electrolyte of Na₂SO₄ solution prior the main pickling in mixed acid. In today state-of-the-art practice, the Mechanical de-scaling (MD) and Pre-Pickling (PP) can remove the oxide scale layer only, the chromiumdepleted zone and partly the base material can be only removed by the final pickling process (FP).

The pickling process most commonly used for stainless steel involves the use of a mixture of nitric and hydrofluoric acid, the mutual concentrations of which vary according to the type of stainless steel to be pickled (austenitic, ferritic, martensitic, duplex . . . ), its surface characteristics and its past processing history. When processing of 300 and 400 series in the same line, various acid compositions (Mix I and Mix II) and various acid mixture temperatures are required. Since austenitic steel grades are pickled at 50-65° C., the most of ferritic and martensitic stainless steel grades generate exothermal reaction during the pickling, which require cooling facilities in the pickling line in order to keep the acid mixture temperature in the range of 35-40° C. .Although the process enables excellent pickling results to be obtained, it has the very serious drawback that it creates considerable and substantial ecological problems due to the use of these particular acids. The hydrofluoric acid is extremely corrosive and a harmful environmental pollutant. The nitric acid is the source of highly polluting nitrogen oxide (NOx) vapors which are emitted into the atmosphere and which are highly aggressive towards metals and nonmetals with which they come into contact. In addition, high nitrate levels exist in the rinse water and in the spent pickling baths and create a major disposal problem. The elimination of NOx vapors in the air by catalytic DENOX plants and nitrates in the neutralized waste water creates considerable plant operational problems, very high investment costs for equipment, high maintenance demand and very high operational costs. As a result, there has been considerable interest in researching and developing stainless steel pickling processes and plants which do not use either nitric acid or hydrofluoric acid and which are ecologically safe and environmental-friendly.

Today pickling lines are designed as shallow tank turbulence installations comprising of several consecutive pickling tanks. The steel strip is pulled or pushed through the treatment tanks. The complete pickling section is arranged as a cascade, i.e. the fresh or regenerated acid is added to the last treatment tank (i.e. the most downstream treatment tank according to the direction of movement of the steel strip) and is then processed in a countercurrent flow to the strip transport direction in order to maximize the use of the pickling acid. At the entry and exit of the treatment tanks, wringer rolls are installed to remove the pickling acid from the strip to the greatest possible extend in order to enhance the cascade effect. German patent disclosure DE 40 31 234 describes this technology.

Inside the treatment tank, the pickling acid is injected on both sides of the tank creating a high turbulence between the strip surface and the pickling acid.

The pickling acid is then overflowing from the treatment tank to a circulation tank from where it is again injected into the treatment tank by means of pumps. The high turbulence reduces the thickness of the liquid boundary layer on the strip surface resulting in an improved media and energy exchange and consequently reducing the required pickling time.

Another well known pickling method is the spray pickling, wherein the pickling acid is directly sprayed onto the strip surface using several spray nozzles installed both above and below the steel strip, cf. e.g. document DE 42 28 808 A1. The pickling acid is then collected in a circulation tank from where it is pumped to the spray nozzles and sprayed on to the strip surface again. The spray nozzles are typically operated at a pressure above 1 bar. Due to the high impulse of the pickling acid sprayed onto the strip surface, the pickling efficiency and consequently the pickling time can be further improved. However this technology has never been used commercially in strip pickling lines.

The use of hydrochloric acid as a pickling agent for stainless steel pickling allows the realization of pickling mechanisms of both the removal of oxide scale and the chromium-depleted zone. The pickling of stainless steel in hydrochloric acid is a combined process of reduction and oxidation. The chemical dissolution of steel in HCl is as follows:

Fe+2 HCl→FeCl₂+H₂

The base metal, Fe, is dissolved by the oxidizing agents, mainly FeCl₃:

2 FeCl₃+Fe→3 FeCl₂

Oxidation reaction to produce the required oxidizing agent is as follows:

4 FeCl₂+O₂+4 HCl→4 FeCl₃+2 H₂O

A minimum proper FeCl₃ concentration is required for the pickling process of stainless steel. This is today typically reached by adding H₂O₂ to the pickling liquid.

In the context of the present invention, chlorides of iron and chlorides of other metals (especially chromium) are collectively referred to by the term MeCl.

Laboratory tests, carried out for different steel grades, have proven for HCl containing pickling solution, that the pickling speed of spray-pickling is up to five times higher compared to the shallow tank turbulence technology. In addition, the spray nozzles used in the spray pickling technology create fine droplets with a high surface which are in direct contact with air. The air, in particular the oxygen contained in the air, dissolves in the pickling acid and oxidizes the FeCl₂ together with the HCl forming FeCl₃. Therefore using HCl in a spray pickling section to treat stainless steel has the advantage, that no H₂O₂ is needed to create FeCl₃. However, in pure spray pickling tanks, the formation of FeCl₃ can be too high (to reach 60 g/L and above), making the whole pickling process difficult to control, with a high risk of over pickling the metal strip or causing inacceptable roughness of the strip surface. Another drawback of the increased FeCl₃ concentration in the pickling acid is the effect on the regeneration process of the spent pickling acid. Spent pickling acid containing HCl is typically regenerated using the pyrohydrolysis process. In this process FeCl₂ and FeCl₃ are converted back to HCl and Fe₂O₃. FeCl₃ however has a much lower evaporation temperature than FeCl₂ and evaporates in the pyrohydrolysis reactor causing very fine Fe₂O₃ particles below 1 μm in size when converted to Fe₂O₃. These fine particles are difficult to remove from the process off-gases causing high dust emissions.

SUMMARY

An aspect of the invention provides a method for treating a steel strip using a first treatment liquid in a first treatment station, the treatment station including a first treatment tank including a first spray section, and a first immersion section, and a first common collector for the first treatment liquid, the steel strip including stainless steel, a top surface, and a bottom surface, and being a continuous steel strip, oriented substantially horizontally, both in its longitudinal and transverse directions. The method comprises: transporting the steel strip continuously through the first treatment station in a transport direction parallel to the longitudinal direction of the steel strip; spraying, using one or more spraying nozzles, the first treatment liquid onto the top and bottom surfaces of the steel strip while the steel strip is in the first spray section; immersing the steel strip in the first treatment liquid while the steel strip is in the first immersion section; and, while treating the steel strip, continuously pumping the first treatment liquid out of the first common collector and through both the first spray section and the first immersion section.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

FIG. 1 schematically, a treatment system comprising three different treatment stations, each one being configured according to the present invention, i.e. having a treatment tank with both a spray section and an immersion section such that a common collection means and the same treatment liquid is used for the treatment of the steel strip in both the respective spray section and the immersion section;

FIG. 2 schematically, a first embodiment of a treatment station having a treatment tank and a common collection means separated from the treatment tank, the treatment tank having its spray section and its immersion section to treat the steel strip with a common treatment liquid circulating between and within the common collection means on the one hand, and the spray and immersion sections on the other hand; and

FIG. 3 schematically, a second embodiment of a treatment station having a treatment tank and a common collection means separated from the treatment tank, the treatment tank having its spray section and its immersion section to treat the steel strip with a common treatment liquid circulating between and within the common collection means on the one hand, and the spray and immersion sections on the other hand.

DETAILED DESCRIPTION

An aspect of the present invention relates to a method for treating a steel strip especially for a pickling treatment of the steel strip, by means of a treatment liquid in a treatment station, the treatment station comprising a treatment tank. Furthermore, an aspect of the present invention relates to a system for treating a steel strip, especially for a pickling treatment of the steel strip, by means of a treatment liquid in a treatment station, the treatment station comprising a treatment tank. The steel strip to be treated according to the method of an aspect of the present invention and in a system of an aspect of the present invention is stainless steel

It is therefore an aspect of the present invention provides a method and a system for an improved steel strip treatment, especially pickling, such that fixed investment as well as maintenance costs are reduced, the treatment and pickling process is realized comparatively quickly, with high quality, and in an environmentally friendly manner.

An aspect of the present invention provides a method for treating a steel strip, especially for a pickling treatment of the steel strip, by means of a treatment liquid in a treatment station, the treatment station comprising a treatment tank with a spray section and an immersion section, and the treatment station comprising a common collection means for the treatment liquid, wherein the steel strip comprises stainless steel and is a continuous steel strip being oriented substantially horizontally, both in its longitudinal and transverse directions, wherein the steel strip has a top surface and a bottom surface, wherein the method comprises transporting the steel strip continuously through the treatment station in a transport direction, the transport direction being parallel to the longitudinal direction of the steel strip, such that—in a first step, the treatment liquid is sprayed onto the top surface of the steel strip and onto the bottom surface of the steel strip while the steel strip being in the spray section of the treatment station,—in a second step, the steel strip is immersed in the treatment liquid while the steel strip being in the immersion section of the treatment station, wherein, while treating the steel strip, the treatment liquid is continuously pumped out of the common collection means and through both the spray section and the immersion section of the treatment station, wherein spraying of the treatment liquid onto the top and bottom surfaces of the steel strip is provided using spray nozzles.

According to an aspect of the present invention, it is thereby advantageously possible to provide a treatment station that requires comparatively low installation costs as well as reduced maintenance costs. An aspect of the present invention is related to a process for chemical or electrochemical treatment of the surface of stainless steel, preferably strip shaped, wherein the material is treated with a pickling solution, preferably containing HCl, in one or more treatment tanks to remove an oxide scale layer which was previously formed during the hot rolling process of the metal strip (steel strip). This treatment is needed to reach a clean surface for either further processing it in a cold rolling process or for direct commercial use.

According to an embodiment of the present invention, instead of the steel strip comprising stainless steel, it is also preferred that the steel strip consists of stainless steel.

According to an aspect of the present invention, it is preferred that the process for chemical or electrochemical treatment of the surface of stainless steel is conducted using a pickling solution containing HCl as the only pickling agent, wherein the advantages of spray pickling are used to a maximum extend. Furthermore the process shall be controllable minimizing the risk of over-pickling so that the process can be realized in commercially used industrial scale pickling lines.

This drastically shortening of the pickling time in case of spray and turbulence pickling can be explained by a very thin laminar boundary layer, which is much more thinner than in case of turbulence dip process. The drive force of heat, mass and momentum transfer across the boundary layer is faster, since the thickness of this layer is significantly reduced.

According to an aspect of the present invention, the spent acid of such a process is of a quality such that it can be treated in regeneration plants without additional investment considering in particular the FeCl₃ concentration in such spent acid.

An aspect of the present invention is also directed to the possibility to revamp existing pickling lines, in particular the treatment tanks, and to use more efficient pickling technology with improved efficiency while re-using existing equipment in order to reduce installation costs, as for example acid circulation circuits etc. can be reused

According to an aspect of the present invention, it is advantageously possible that such requirements can be achieved by the present invention, comprising a pickling process using an HCl containing pickling solution as the only pickling acid, wherein the material to be treated (i.e. the steel strip) is processed horizontally through one or more treatment tanks which are—in case of more than one treatment tank—operated as a pickling cascade.

According to an aspect of the present invention, each single treatment tank (of the treatment station) of the above described process comprises of one spray pickling zone and one dip pickling zone arranged as one unit using one common circulation circuit, i.e. one common circulation tank (common collection means) with several pump circuits as required. All pickling acid coming from the dip section and the spray section are collected and mixed in the common circulation tank (common collection means) and pumped back to the above mentioned two pickling sections (of the treatment tank of the treatment station). Inside the single pickling tank (treatment tank), a guide roll underneath the strip located between the spray and pickling section might be required to better position the steel strip inside the treatment tank. Typically, a wringer roll unit—as it is typically installed between two pickling sections—is not required. Preferably the first section of the treatment tank is a spray section while the second section of the dip pickling type, preferably with high efficiency such as shallow tank turbulence technology.

According to an aspect of the present invention, the steel strip is treated—in the treatment tank of the treatment station—by means of a treatment liquid such that the same treatment liquid is used both in the spray section and in the immersion section of the treatment tank. Advantageously, it is thereby possible to realize the treatment station (having both the spray section and the immersion section) in a more cost effective manner as the same common collection means (as well as at least a part of the circulation system) can be used for both the spray section and the immersion section, hence reducing the costs for realizing the possibility to treat the steel strip by means of both the spray section and the immersion section.

According to an aspect of the present invention, the steel strip comprises stainless steel and is a continuous steel strip being oriented substantially horizontally, both in its longitudinal and transverse directions, at least at the treatment station. This means that steel strip is mostly horizontally oriented in its transverse direction but is allowed to be sagging in its longitudinal direction. The height variation through the treatment station of the steel strip in its longitudinal direction may reach, e.g., up to 0.5 m. Preferably, also between the treatment station or between the plurality of treatment stations, the height variations of the steel strip in its longitudinal direction are also comprised up to 0.5 m. Generally, it is preferred according to the present invention that the height variation of the steel strip in its longitudinal direction is comprised between up to 0.5 m throughout the complete treatment system, that potentially (and typically) comprises a plurality of treatment stations one after the other in the transport direction of the steel strip.

According to an aspect of the present invention, the treatment liquid is sprayed—in a first step and by means of nozzles—onto the top surface of the steel strip and onto the bottom surface of the steel strip while the steel strip is in the spray section of the treatment station. In second step (that is not necessarily subsequent to the first step but could also be preceding the first step), the steel strip is immersed in the treatment liquid while the steel strip is in the immersion section of the treatment station. For the treatment of the steel strip, the treatment liquid of the treatment station is continuously pumped out of the common collection means (of that treatment station) and through both the spray section and the immersion section of the treatment tank, wherein spraying of the treatment liquid onto the top and bottom surfaces of the steel strip is provided using spray nozzles.

According to an aspect of the present invention, two pickling technologies are directly combined in one treatment tank (i.e. in one treatment station), i.e. using physically the same pickling acids (or the same treatment liquid) in both pickling sections (i.e. in both the spray section and the immersion section of the considered treatment station), as described. By doing so, the concentration of FeCl₃ can be kept below a critical level throughout the entire pickling process, guaranteeing a uniform pickling result without the risk of over-pickling. Furthermore the spent acid of such process can be easily regenerated in regeneration plants without additional investment to reach the legally required emission values, especially regarding dust emissions.

According to an aspect of the present invention, the efficiency of the treatment process (or pickling process) is increased. Tests have proven that a certain increase in the FeCl₃ concentration reduces the pickling time also for the dip pickling process. Consequently the process according to the present invention uses the advantage of the high efficient spray pickling process while the efficiency of the dip pickling process is improved as well, due to the common use of the pickling acid (i.e. the same treatment liquid is used both in the spray section and the immersion section of one and the same treatment station), and the consequently increased FeCl₃ level. Of course, in (the typical) case that more than one treatment stations are used in a pickling line or steel strip pickling installation, this does not mean that the same treatment liquid is used in all of such different treatment stations. To the contrary, in case of a plurality of treatment stations (i.e. having each a treatment tank comprising a spray section and an immersion section), a different treatment liquid is normally used for a different treatment station; however within the same treatment station/treatment tank, the same treatment liquid is used for both kinds of pickling processes (spray and dip pickling). Thereby, it is advantageously possible that the drawbacks of a comparatively high concentration in FeCl₃ can be avoided that would typically arise in case of combining spray pickling and dib pickling using different treatment liquids in the same treatment station.

According to an aspect of the present invention, the design of the treatment line or pickling line is done in such a way that it is advantageously possible that the treatment stations or treatment tanks can easily replace existing treatment tanks in case of a required revamp (or refurbishment) while the circulation circuits can be reused. This is mainly attributed to the fact that the spray pickling technology and the dip pickling technology (i.e. the spray section and the immersion section) are combined in one treatment tank (i.e. as part of one treatment tank).

The design of an aspect of the present invention also allows the possibility to operate the treatment tanks without an additional (external) circulation tank—or common collection means—(i.e. external or separate to the treatment tank). In such an embodiment, the treatment tank itself, in particular the area underneath the spray section, and, if required, also underneath the dip section, is used as circulation tank (or common collection means), i.e. the circulation tank (or common collection means) is realized in a manner integrated with the treatment tank. This is advantageous for the replacement (refurbishment) of deep bath treatment tanks which have often been operated without circulation circuits. In this case only the pump circuit needs to be added while the circulation tank is incorporated (or integrated) in the treatment tank.

According to a preferred embodiment of the present invention, the spray section comprises an effective spray length in parallel to the longitudinal direction of the steel strip such that—during the first step—the top and bottom surfaces of the steel strip receive the treatment liquid while being located within the effective spray length, wherein the immersion section comprises an effective immersion length in parallel to the longitudinal direction of the steel strip such that—during the second step—the steel strip is immersed—with its top and bottom surfaces—in the treatment liquid while being located within the effective immersion length, wherein the effective spray length and the effective immersion length are provided having a ratio of between and including 30:70 to 70:30, especially a ratio of 50:50.

According to an aspect of the present invention, it is thereby advantageously possible to flexibly adapt process parameters of a pickling line to fit best with the intended operative use after construction. By means of defining the length of the immersion section (at a given transport speed of the steel strip through the pickling line), the time is defined during which the treatment liquid is effectively treating the steel strip in the immersion section. By means of defining the length of the spray section (equally at a given transport speed of the steel strip trough the pickling line), the maximum time of spray pickling is defined in relation to the dip pickling time.

According to another preferred embodiment of the present invention, the effective spray length and hence the ratio of the effective spray length vs. the effective immersion length is varied by activating only a part of the spray nozzles.

According to an aspect of the present invention, it is thereby advantageously possible to vary the spray pickling time even during operational use of the pickling line, i.e. by de-activating a part of the spray nozzles. By selectively activating and/or de-activating groups of spray nozzles, is it advantageously possible according to the present invention, that also the manner or the intensity of the spray pickling step can be varied in operational use of the pickling line, e.g. by using only every second spray nozzle such that spray pickling is less intensive in the spray section.

According to a preferred embodiment of the present invention, the spray section is located—along the transport direction of the steel strip—upstream with respect to the immersion section. According to an alternative preferred embodiment of the present invention, the spray section is located—along the transport direction of the steel strip—downstream with respect to the immersion section.

According to an aspect of the present invention, it is thereby advantageously possible to provide the possibility of different pickling line architectures. E.g., it is advantageously possible (in case that at least two treatment stations are used) to provide both treatment stations such that the spray section is located upstream with respect to the immersion section (i.e. the steel strip passes the spray section first and afterwards the immersion section): This results in a pickling sequence of the kind of a spray and dip pickling (using a first treatment liquid) in the first (or upstream) treatment station, followed by a spray and dip pickling (using a second treatment liquid) in the second (or downstream) treatment station. Alternatively, it is also advantageously possible (in case that at least two treatment stations are used) to provide the first treatment station such that the spray section is located downstream with respect to the immersion section (i.e. the steel strip passes the immersion section first and afterwards the spray section), and to provide the second treatment station such that the spray section is located upstream with respect to the immersion section (i.e. the steel strip passes the spray section (of the second treatment station) first and afterwards the dip section (of the second treatment station)): This results in a pickling sequence of the kind of a dip and spray pickling (using a first treatment liquid) in the first (or upstream) treatment station, followed by a spray and dip pickling (using a second treatment liquid) in the second (or downstream) treatment station. Of course, these building blocks of two treatment stations can be either repeated or combined with other treatment stations or configurations of treatment stations.

According to a preferred embodiment of the present invention, the method comprises using—besides using the treatment liquid in the treatment station—a further treatment liquid in a further treatment station, the further treatment station comprising a further treatment tank with a further spray section and a further immersion section, and the further treatment station comprising a further common collection means for the further treatment liquid, wherein the method comprises transporting the steel strip continuously through the further treatment station in the transport direction such that—in a third step, the further treatment liquid is sprayed onto the top surface of the steel strip and onto the bottom surface of the steel strip while the steel strip being in the further spray section of the further treatment station,—in a fourth step, the steel strip is immersed in the further treatment liquid while the steel strip being in the further immersion section of the further treatment station, wherein, while treating the steel strip, the further treatment liquid is continuously pumped out of the further common collection means and through both the further spray section and the further immersion section of the further treatment station, wherein spraying of the further treatment liquid onto the top and bottom surfaces of the steel strip is provided using further spray nozzles, wherein the third and fourth steps are preceding the first and second steps or are subsequent to the first and second steps.

According to an aspect of the present invention, it is thereby advantageously possible to combine at least two inventive treatment stations in a pickling line. Of course, it is also possible and preferred according to the present invention to combine such two inventive treatment stations with a conventional treatment station (i.e. having solely a spray section or solely an immersion section in a treatment tank) or with a plurality of conventional treatment stations. In such an architecture of the pickling line, the two inventive treatment station are either located directly subsequent one after the other along the transport direction of the steel strip or the combination with one or a plurality of conventional treatment stations is provided such that the treatment station (or the first treatment station) is located upstream according to the transport direction of the steel strip with respect to a conventional treatment station (or with respect to a plurality of conventional treatment stations) and downstream with respect to this or these conventional treatment station(s) is located the further treatment station (or second treatment station) according to the present invention.

According to another preferred embodiment of the present invention, the treatment liquid and/or the further treatment liquid comprises—hydrochloric acid in a concentration ranging from and including 150 g/L to and including 250 g/L and—FeCl₃ in a concentration ranging from and including 10 g/L to and including 35 g/L, especially in a concentration ranging from and including 15 g/L to and including 30 g/L or especially in a concentration ranging from and including 19 g/L to and including 26 g/L and,—MeCl₂ in a concentration ranging from and including 30 g/L to and including 300 g/L, especially in a concentration ranging from and including 30 g/L to and including 60 g/L or in a concentration ranging from and including 130 g/L to and including 180 g/L or in a concentration ranging from and including 230 g/L to and including 300 g/L.

According to an aspect of the present invention, it is thereby advantageously possible to combine a high efficiency of the pickling process while retaining the possibility to comparatively easily regenerated the used pickling acids (treatment liquids).

An aspect of the present invention also relates to a system for treating a steel strip, especially for a pickling treatment of the steel strip, by means of a treatment liquid in a treatment station, the system comprising the treatment station, wherein the treatment station comprises a treatment tank with a spray section, an immersion section, and the treatment station comprising a common collection means for the treatment liquid, wherein the steel strip comprises stainless steel and is a continuous steel strip being oriented substantially horizontally, both in its longitudinal and transverse directions, wherein the steel strip has a top surface and a bottom surface, wherein the system is configured to transport the steel strip continuously through the treatment station in a transport direction, the transport direction being parallel to the longitudinal direction of the steel strip, such that—the treatment liquid is sprayed onto the top surface of the steel strip and onto the bottom surface of the steel strip while the steel strip being in the spray section of the treatment station,—the steel strip is immersed in the treatment liquid while the steel strip being in the immersion section of the treatment station, wherein the system is configured such that the treatment liquid is continuously pumped out of the common collection means and through both the spray section and the immersion section of the treatment station, wherein the system comprises spray nozzles such that the treatment liquid is sprayed onto the top and bottom surfaces of the steel strip using the spray nozzles.

According to an aspect of the present invention, it is thereby advantageously possible to provide a system (or a treatment station) that requires comparatively low installation costs as well as reduced maintenance costs. According to the present invention, it is advantageously possible to combine the advantages of spray pickling and dip pickling and to minimize the risk of over-pickling. It is furthermore advantageous that the spent acid of such a system is of a quality such that it can be treated in regeneration plants without additional investment considering in particular the FeCl₃ concentration in such spent acid.

According to an embodiment of the present invention, instead of the steel strip comprising stainless steel, it is also preferred that the steel strip consists of stainless steel.

According to a preferred embodiment of the present invention—especially regarding the inventive system—, the spray section comprises an effective spray length in parallel to the longitudinal direction of the steel strip such that the top and bottom surfaces of the steel strip receive the treatment liquid while being located within the effective spray length, wherein the immersion section comprises an effective immersion length in parallel to the longitudinal direction of the steel strip such that the steel strip is immersed—with its top and bottom surfaces—in the treatment liquid while being located within the effective immersion length, wherein the effective spray length and the effective immersion length are provided having a ratio of between and including 30:70 to 70:30, especially a ratio of 50:50.

According to a preferred embodiment of the present invention—especially regarding the inventive system—, the spray section is located—along the transport direction of the steel strip—upstream with respect to the immersion section. According to an alternative preferred embodiment of the present invention—especially regarding the inventive system—, the spray section is located—along the transport direction of the steel strip—downstream with respect to the immersion section.

According to an aspect of the present invention, it is thereby advantageously possible to flexibly adapt process parameters of a pickling line to fit best with the intended operative use after construction.

According to a preferred embodiment of the present invention—especially regarding the inventive system—, the common collection means for the treatment liquid of both the spray section and the immersion section is a collection means separated from the treatment tank of the treatment station.

According to an aspect of the present invention, it is thereby advantageously possible to build the treatment tank in a very cost effective manner such that especially the volume of the treatment tank is comparably small (and hence less treatment liquid is to be used). The treatment liquid is pumped through the common collection means (or circulation tank) that is separated from the treatment tank.

According to a preferred embodiment of the present invention—especially regarding the inventive system—, the common collection means for the treatment liquid of both the spray section and the immersion section is a collection means integrated with the treatment tank of the treatment station, especially integrated such that the bottom part of the treatment tank forms the common collection means.

According to an aspect of the present invention, it is thereby advantageously possible to realize the treatment station in a very cost effective manner as no separate common collection means (or circulation tank) is required.

According to a preferred embodiment of the present invention—especially regarding the inventive system—, the system comprises—besides the treatment liquid in the treatment station—a further treatment liquid in a further treatment station, the further treatment station comprising a further treatment tank with a further spray section and a further immersion section, and the further treatment station comprising a further common collection means for the further treatment liquid, wherein the system is configured such that the steel strip is transported continuously through the further treatment station in the transport direction such that—the further treatment liquid is sprayed onto the top surface of the steel strip and onto the bottom surface of the steel strip while the steel strip being in the further spray section of the further treatment station,—the steel strip is immersed in the further treatment liquid while the steel strip being in the further immersion section of the further treatment station, wherein the system is configured such that the further treatment liquid is continuously pumped out of the further common collection means and through both the further spray section and the further immersion section of the further treatment station, wherein the system comprises further spray nozzles such that the further treatment liquid is sprayed onto the top and bottom surfaces of the steel strip using the further spray nozzles.

According to an aspect of the present invention, it is thereby advantageously possible to combine at least two inventive treatment stations in a pickling line. Of course, it is also possible and preferred according to the present invention to combine such two inventive treatment stations with a conventional treatment station (i.e. having solely a spray section or solely an immersion section in a treatment tank) or with a plurality of conventional treatment stations.

According to a preferred embodiment of the present invention, the system comprises—besides the treatment liquid in the treatment station and the further treatment liquid in the further treatment station—a third treatment liquid in a third treatment station, the third treatment station comprising a third treatment tank with a third spray section and a third immersion section, and the third treatment station comprising a third common collection means for the third treatment liquid.

According to an aspect of the present invention, it is thereby advantageously possible to combine at least three inventive treatment stations in a pickling line. Of course, it is also possible and preferred according to the present invention to combine such three inventive treatment stations with a conventional treatment station (i.e. having solely a spray section or solely an immersion section in a treatment tank) or with a plurality of conventional treatment stations.

According to a further preferred embodiment of the present invention, the system comprises—besides the treatment liquid in the treatment station, the further treatment liquid in the further treatment station, and the third treatment liquid in the third treatment station—a fourth treatment liquid in a fourth treatment station, the fourth treatment station comprising a fourth treatment tank with a fourth spray section and a fourth immersion section, and the fourth treatment station comprising a fourth common collection means for the fourth treatment liquid. According to other embodiments, also the combination of five treatment stations according to the present invention is possible and preferred according to the present invention.

According to another preferred embodiment of the present invention, the treatment liquid and/or the further treatment liquid and/or the third treatment liquid comprises—hydrochloric acid in a concentration ranging from and including 150 g/L to and including 250 g/L and—FeCl₃ in a concentration ranging from and including 10 g/L to and including 35 g/L, especially in a concentration ranging from and including 15 g/L to and including 30 g/L or especially in a concentration ranging from and including 19 g/L to and including 26 g/L and,—FeCl2 in a concentration ranging from and including 30 g/L to and including 300 g/L, especially in a concentration ranging from and including 30 g/L to and including 60 g/L or in a concentration ranging from and including 130 g/L to and including 180 g/L or in a concentration ranging from and including 230 g/L to and including 300 g/L.

According to an aspect of the present invention, it is thereby advantageously possible to combine a high efficiency of the pickling process while retaining the possibility to comparatively easily regenerated the used pickling acids (treatment liquids).

These and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Where an indefinite or definite article is used when referring to a singular noun, e.g. “a”, “an”, “the”, this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described of illustrated herein.

FIG. 1 schematically illustrates a treatment system comprising three different treatment stations 3, 31, 32 as an example of a pickling line according to the present invention. In one possible and preferred implementation of the pickling line for the treatment of a steel strip 1, represented in FIG. 1, all three treatment stations 3, 31, 32 represent treatment stations according to the present invention, i.e. having a treatment tank with both a spray section and an immersion section such that a common collection means and the same treatment liquid is used for the treatment of the steel strip in both the respective spray section and the immersion section. In this implementation or architecture of the pickling line, all three treatment stations 3, 31, 32 are realized either according to a first embodiment of the present invention, represented in FIG. 2 for the exemplary case of the treatment station being represented by reference sign 3, or according to a second embodiment of the present invention, represented in FIG. 3 for the exemplary case of the treatment station being represented by reference sign 3. Alternatively, a part of the three treatment stations 3, 31, 32 is or are realized according to the first embodiment of the present invention (FIG. 2) and another part is or are realized according to the second embodiment of the present invention (FIG. 3). In the context of the present invention, the terms ‘treatment station’ and ‘first treatment station’ as well as ‘further treatment station’ and ‘second treatment station’ are used synonymously and only aim to differentiate the treatment stations from one another. Typically, the naming convention typically (but not necessarily) relates to the location of a treatment station along the transport direction of the steel strip, the transport direction being represented by reference sign 2. In the implementation represented in FIG. 1, a treatment station (or first treatment station) 3 is located upstream of a further treatment station (or second treatment station) 31. The further treatment station (or second treatment station) 31 is located upstream of a third treatment station 32. The treatment station (or first treatment station) 3 comprises a treatment tank (or first treatment tank) 4, and a common collection means (or first common collection means) 5. The further treatment station (or second treatment station) 31 comprises a further treatment tank (or second treatment tank) 41, and a further common collection means (or second common collection means) 51. The third treatment station 32 comprises a third treatment tank 42, and a third common collection means 52. In the exemplary implementation of the pickling line represented in FIG. 1 (where all treatment stations are configured according to the present invention), all tree treatment stations 3, 31, 32 each have a spray section and an immersion section as part of their respective treatment tank 4, 41, 42, i.e. the treatment station (or first treatment station) 3 has a spray section (or first spray section) and an immersion section (or first immersion section) using a treatment liquid (or first treatment liquid), the further treatment station (or second treatment station) 31 has a further spray section (or second spray section) (not depicted in FIG. 1) and a further immersion section (or second immersion section) (not depicted in FIG. 1) using a further treatment liquid (or second treatment liquid), and the third treatment station 32 has a third spray section (not depicted in FIG. 1) and a third immersion section (not depicted in FIG. 1) using a third treatment liquid. For the exemplary case of the treatment station 3 (first treatment station 3), a first and a second embodiment of the present invention is schematically shown in FIGS. 2 and 3.

FIG. 2 schematically illustrates the first embodiment of the treatment station 3 having the treatment tank 4 and the common collection means 5 separated from the treatment tank 4, the treatment tank 4 having its spray section 13 and its immersion section 14 to treat the steel strip 1 with a common treatment liquid circulating between—and within—the common collection means 5 on the one hand, and the spray and immersion sections 13, 14 on the other hand.

FIG. 3 schematically illustrates a second embodiment of a treatment station 3 having the treatment tank 4 and the common collection means 5 separated from the treatment tank 4, the treatment tank 4 having its spray section 13 and its immersion section 14 to treat the steel strip 1 with a common treatment liquid circulating between—and within—the common collection means 5 on the one hand, and the spray and immersion sections 13 on the other hand.

FIGS. 1, 2 and 3 combined illustrate the inventive treatment process and system (or treatment station) for chemical or electrochemical treatment of the surface of the steel strip 1, the steel strip 1 being a stainless steel strip. The steel strip 1 is first horizontally transported through the treatment stations 3, 31, 32 in which the steel strip 1 is treated with a treatment liquid in the form of a pickling acid, normally containing HCl. At least one of the treatment stations 3, 31, 32 (or their respective treatment tanks 4, 41, 42) comprises a spray pickling section (as represented in FIGS. 2 and 3 as spray section 13 of the treatment station 3) and a dip pickling section (as represented in FIGS. 2 and 3 as immersion section 14 of the treatment station 3) according to the present invention. FIG. 1 shows an exemplary implementation with three treatment station 3, 31, 32 (each having a treatment tank) as a preferred embodiment, however the number of treatment stations (and treatment tanks) is at least one and is not limited to three.

All treatment stations 3, 31, 32 comprise a common collection means, respectively (i.e. the respective treatment tanks 4, 41, 42 are connected to respective common collection means (or circulation tanks) 5, 51, 52), wherein the common collection means 5, 51, 52 are either (i.e. potentially for each treatment station 3, 31, 32 differently) realized as separate tanks as shown in the first embodiment represented in FIG. 2 or are realized as common collection means 5, 51, 52 integrated in the respective treatment tank 4, 41, 42 as shown in FIG. 3.

In the exemplary embodiment shown in FIG. 1, the common collection means (or circulation tanks) 5, 51, 52 are operated as a cascade, i.e. the fresh or regenerated acid (i.e. the treatment liquid) is added (cf. reference sign 54) to the last common collection means (or last circulation tank) 52—i.e. being related to the most downstream treatment station 32 according to the transport direction 2 of the steel strip 1—and is consequently transferred to the other common collection means (or circulation tanks) in counter direction to the strip transport direction 2. Thereby, the level of free acid is the highest in the third treatment liquid (circulating in the third treatment station 32), the level of free acid is medium in the further treatment liquid (second treatment liquid) (circulating in the further (second) treatment station 31), and lowest in the treatment liquid (first treatment liquid) (circulating in the (first) treatment station 3. Finally the spent acid is removed (reference sign 55) from the (first) common collection means (or (first) circulation tank) 5. After the pickling treatment in the three treatment stations 3, 31, 32 (in the exemplary implementation shown in FIG. 1), the steel strip 1 is further processed in section 6 which comprises a rinse section and a dryer, if required.

According to the first embodiment of the common collection means (or circulation tank) 5, shown in FIG. 2, the treatment station 3 comprises the treatment tank 4 with a separate common collection means 5 (or separate circulation tank 5). At the entry and exit section, wringer rolls 12 are installed to remove pickling acid from the strip and to guide the steel strip 1 inside the treatment tank 4. The wringer roll 12 in the entry section is only used when the treatment tank is the first tank in the pickling process like the treatment station 3 in FIG. 1. The following treatment stations (or treatment tanks), like treatment stations 31, 32 in FIG. 1, do not need such wringer roll 12. In the exemplary representation of FIG. 2 (i.e. not necessarily), the first part (according to the transport direction of the steel strip 1) of the treatment tank 4 is a spray pickling section 13 or spray section 13, followed by a dip pickling section 14 or immersion section 14. In the spray pickling section 13, spray nozzles 15 are mounted above and below the surface of the steel strip 1. The pickling acid (or treatment liquid) is pumped from the circulation tank 5 (or common collection means 5) by means of pumps 17, 18 to both the spray pickling section 13 and the dip pickling section 14. In one or more of the pump circuits, a heat exchanger 19 is installed to heat the pickling acid (treatment liquid) to the required temperature. If required, a guide roll 20 can be installed between the spray pickling section 13 and the dip pickling section 14 in order to reduce the slack of the strip.

According to the second embodiment of the common collection means (or circulation tank) 5, shown in FIG. 3, the treatment station 3 comprises the treatment tank 4 with an integrated common collection means 5. The other components of the treatment station 3 are analogous to the description of FIG. 2.

While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

For example, the pickling line is configured for a maximum width of the steel strip 1 of 1890 mm, a maximum speed of the steel strip 1 of 85 m/min. Furthermore exemplarily, the distance of the spray nozzles 15 to the steel strip 1 (both from the spray nozzles to the top surface 1′ of the steel strip 1, and to the bottom surface 1″ of the steel strip 1) is 200 mm or approximately 200 mm. Additionally, the distance of the spray nozzles 15 to each other in the lateral direction of the steel strip 1 corresponds to 200 mm or approximately 200 mm. Additionally, the distance of the spray nozzles 15 to each other in the longitudinal direction of the steel strip 1 corresponds to 500 mm or approximately 500 mm. The treatment liquid is preferably pumped out of the spray nozzles having a pressure of between and including 1 bar to and including 3 bar, and the amount of treatment liquid per spray nozzle is preferably 12 l/min or approximately 12 l/min. For example, the total number of spray nozzles per treatment station corresponds to 306 or approximately 306, and the amount of pumped treatment liquid per treatment station corresponds to 220 m³/h or approximately 220 m³/h.

Test trials were carried out in a pilot plant. The pilot plant consisted of two treatment stations (each having a treatment tank) both arranged as described in the present invention with a first spray pickling section followed by a dip pickling section in each of the treatment tanks. The treatment tanks were designed so that the length of both sections was approximately the same. The pickling acid used was HCl with a concentration of approx. 200 g/L total acid in both tanks. The material treated during the test runs were different austenitic steel grades such as AISI 304 and 316. The test results have proven that the pickling time could be reduced by 40-45% compared to the conventional pickling process using dip pickling with shallow tank turbulence technology, while the FeCl₃ concentration was constantly below 30 g/L which is considered to be uncritical as far as the acid regeneration process is concerned. All tested materials showed uniform pickling results without any signs of over-pickling.

In another test using the same pilot plant, the material was treated with a reduced temperature of the pickling acid (treatment liquid). The results showed that the temperature could be reduced from 90° C. down to 70° C. while still reaching the same pickling time as for the conventional dip pickling process with shallow tank turbulence technology. This result is equivalent to a 20% reduction of the energy which is needed to keep the process temperature in the pickling process.

As an example of the operation of the system and especially of the use of the treatment liquids as a cascade, an example is given of the concentration values for an example of using three treatment stations in a pickling line:

In the first treatment station 3, the concentration of HCl is in the range of between and comprising 201 g/L to and comprising 215 g/L, the concentration of MeCl₂ is in the range of between and comprising 270 g/L to and comprising 286 g/L, the concentration of FeCl₃ is in the range of between and comprising 23 g/L to and comprising 29 g/L. The temperature of the treatment liquid is in the range of between and comprising 87° C. to and comprising 89° C.

In the second treatment station 31, the concentration of HCl is in the range of between and comprising 204 g/L to and comprising 214 g/L, the concentration of MeCl₂ is in the range of between and comprising 141 g/L to and comprising 149 g/L, the concentration of FeCl₃ is in the range of between and comprising 19 g/L to and comprising 23 g/L. The temperature of the treatment liquid is in the range of between and comprising 91° C. to and comprising 93° C.

In the third treatment station, the concentration of HCl is in the range of between and comprising 190 g/L to and comprising 201 g/L, the concentration of MeCl₂ is in the range of between and comprising 40 g/L to and comprising 50 g/L, the concentration of FeCl₃ is in the range of between and comprising 20 g/L to and comprising 22 g/L. The temperature of the treatment liquid is in the range of between and comprising 88° C. to and comprising 91° C.

Various tests were made with different qualities of the hot rolled stainless steel strips. Tests were done majority with austenitic AISI 304 and 316 grades as well as with ferritic AISI 409 and 430 grades, but also with other steel grades. The best results were achieved with steel that has been either shot-blasted or the scale-broken off prior to pickling in hydrochloric acid. In these trials some of the stainless steel grades were compared under the same pickling conditions. In all trials combined spray pickling and dip pickling in one treatment step using a common circulation system gave the shortest pickling time and excellent surface quality while the FeCl₃ concentration could be kept at a level below 30 g/L Significantly reduction of the pickling time were achieved for heavy to pickle steel grades as this is the case of coils coiled at higher coiling temperature e.g. >700° C.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.

The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B, and C” should be interpreted as one or more of a group of elements consisting of A, B, and C, and should not be interpreted as requiring at least one of each of the listed elements A, B, and C, regardless of whether A, B, and C are related as categories or otherwise. Moreover, the recitation of “A, B, and/or C” or “at least one of A, B, or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B, and C.

REFERENCE SIGNS

1 steel strip

2 transport direction of the steel strip

3 treatment station

4 treatment tank (of the treatment station)

5 common collection means (of the treatment station)

12 wringer roll(s)

13 spray section (of the treatment station)

14 immersion section (of the treatment station)

15 spray nozzles (of the treatment station)

17, 18 pumps

19 heat exchanger

20 guide roll(s)

31 further treatment station

41 further treatment tank (of the further treatment station)

51 further common collection means (of the further treatment station)

32 third treatment station

42 third treatment tank (of the third treatment station)

52 third common collection means (of the third treatment station)

54 feeding of fresh treatment liquid

55 removing of used treatment liquid 

1. A method for treating a steel strip, using a first treatment liquid in a first treatment station, the treatment station including a first treatment tank including a first spray section, and a first immersion section, and a first common collector for the first treatment liquid, the steel strip including stainless steel, a top surface, and a bottom surface, and being a continuous steel strip, oriented substantially horizontally, both in its longitudinal and transverse directions, the method comprising: transporting the steel strip continuously through the first treatment station in a transport direction parallel to the longitudinal direction of the steel strip; spraying, using spraying nozzles, the first treatment liquid onto the top and bottom surfaces of the steel strip while the steel strip is in the first spray section; immersing the steel strip in the first treatment liquid while the steel strip is in the first immersion section; and while treating the steel strip, continuously pumping the first treatment liquid is out of the first common collector and through both the first spray section and the first immersion section.
 2. The method of claim 1, wherein the first spray section includes an effective spray length in parallel to the longitudinal direction of the steel strip such that, during the spraying, the top and bottom surfaces of the steel strip receive the first treatment liquid while located within the effective spray length, wherein the first immersion section includes an effective immersion length in parallel to the longitudinal direction of the steel strip such that, during the immersing, the steel strip is immersed, with its top and bottom surfaces, in the first treatment liquid while located within the effective immersion length, wherein the effective spray length and the effective immersion length have a ratio in a range of from 30:70 to 70:30.
 3. The method of claim 2, wherein the effective spray length and hence the ratio of the effective spray length versus the effective immersion length is varied by activating, only a part of the spray nozzles.
 4. The method of claim 1, wherein, along the transport direction of the steel strip, the first spray section is located upstream or downstream of the first immersion section.
 5. The method of claim 1, further comprising, using a second treatment liquid in a second treatment station including a second treatment tank including a second spray section, a second immersion section, and a second common collector for the further treatment liquid; transporting the steel strip continuously through the second treatment station in the transport direction; spraying, using spray nozzles, the second treatment liquid onto the top and bottom surfaces of the steel strip while the steel strip is in the second spray section; immersing the steel strip in the second treatment liquid while the steel strip is in the second immersion section of the further treatment station; and while treating the steel strip, continuously pumping the second treatment liquid out of the second common collector and through both the second spray section and the second immersion section, wherein the spraying and immersing in the second treatment liquid precede or follow the spraying and immersing in the first treatment liquid.
 6. The method of claim 5, wherein the first treatment liquid and/or the second treatment liquid comprises hydrochloric acid in a concentration range of from 150 g/L to 250 g/L. FeCl₃ in a concentration range of from 10 g/L to 35 g/L, MeCl₂ in a concentration range from 30 g/L to 300 g/L.
 7. A system for treating a steel strip including stainless steel, and having a top surface, and a bottom surface, and being a continuous steel strip, oriented substantially horizontally, both in its longitudinal and transverse directions, using a treatment liquid in a treatment station, the system comprising: a treatment station including a treatment tank including a spray section, an immersion section, and a common collector for the treatment liquid: and spray nozzles configured to spray that the treatment liquid onto the top and bottom surfaces of the steel strip, wherein the system is configured to transport the steel strip continuously through the treatment station in a transport direction, the transport direction being parallel to the longitudinal direction of the steel strip, such that the treatment liquid is sprayed onto the top and bottom surfaces of the steel strip while the steel strip is in the spray section, and the steel strip is immersed in the treatment liquid while the steel strip is in the immersion section, wherein the system is configured such that the treatment liquid is continuously pumped out of the common collector and through both the spray section and the immersion section of the treatment station.
 8. The system of claim 7, wherein the spray section includes an effective spray length in parallel to the longitudinal direction of the steel strip such that the top and bottom surfaces of the steel strip receive the treatment liquid While being-located within the effective spray length, wherein the immersion section includes an effective immersion length in parallel to the longitudinal direction of the steel strip such that the steel strip is immersed, with its top and bottom surfaces, in the treatment liquid while located within the effective immersion length, wherein the effective spray length and the effective immersion length have a ratio in a range of from 30:70 to 70:30.
 9. The system according to one of claim 7, wherein, along the transport direction of the steel strip, the spray section is located upstream or downstream of the immersion section.
 10. The system of claim 7, wherein the common collector is separated from the treatment tank of the treatment station.
 11. The system of claim 7, wherein the common collector is integrated with the treatment tank.
 12. The system of claim 7, further comprising: a second treatment liquid in a second treatment station including a second treatment tank including a second spray section, a second immersion section, and a second common collector for the second treatment liquid; and second spray nozzles configured to spray the second treatment liquid onto the top and bottom surface of the steel strip, wherein the system is configured such that the steel strip is transported continuously through the second treatment station in the transport direction such that the second treatment liquid is sprayed onto the top and bottom surfaces of the steel strip while the steel strip is in the second spray section, and such that the steel strip is immersed in the second treatment liquid while the steel strip is in the second immersion section, wherein the system is configured such that the second treatment liquid is continuously pumped out of the second common collector and through both the second spray section and the second immersion section.
 13. The system of claim 11, further comprising: a third treatment liquid in a third treatment station including a third treatment tank including a third spray section, a third immersion section, and a third common collector for the third treatment liquid.
 14. The system of claim 12, wherein the treatment liquid and/or the second treatment liquid and/or the third treatment liquid comprises hydrochloric acid in a concentration range of from 150 g/L to 250 g/L, FeCl₃ in a concentration range of from 10 g/L to 35 g/L, and MeCl₂ in a concentration range of from 30 g/L to 300 g/L. 